A boiler is an apparatus in which water or some other aqueous temperature control liquid to which makeup water is added and from which blowdown is removed is vaporized into steam by the application of heat from a furnace or heat-generating process system. In most instances, the temperature control liquid is brought into close, indirect contact with the process system to facilitate heat transfer. Leakage in a boiler can result not only in contamination and fouling of the temperature control liquid and the process system, but also in undesired physical reactions. This is particularly true for the black liquor recovery boilers used in many paper mills. In black liquor recovery boilers, the escape or leakage of aqueous temperature control liquid from the so-called "water side" of the boiler into the hot, highly caustic "fire side" can result in violent explosions.
The prior art provides numerous techniques for monitoring and controlling leaks in black liquor recovery boilers and other boiler systems. For example, U.S. Pat. No. 5,320,967 (Avallone, et al.) discloses a boiler system leak detection method that involves introducing an inert tracer to the boiler in a known and uniform proportion to the feedwater, sensing a characteristic of the tracer in the boiler at steady state, converting the sensed characteristic to a value equivalent to the concentration of the tracer in the temperature control liquid, and activating a signal when there is excessive variance in the concentration of the tracer. However, the method disclosed by Avallone, et al. is limited by its requirement that the tracer be detected (sensed) when the boiler is at steady state, which is said to occur only when there is no significant change in any of five process parameters: the concentration of the tracer in the boiler; the blowdown rate; the feedwater rate; the rate of feeding tracer to the boiler; and the steaming rate in the absence of boiler leakage.
Further limitations include the costs of tracer chemicals and measuring equipment for both inputting tracer chemicals and for analyzing blowdown.
U.S. Pat. No. 5,363,693, Nevruz, teaches methods and apparatus for detecting leakage from chemical recovery boiler systems. The methods utilize measuring the mass input and output of a recovery boiler and calculating the long and short term statistics for the drum balance of mass flow. From these calculations a t-test function is calculated to see if both long term and short term moving average of drum balances are significantly different, which in turn indicates whether a boiler leak is occurring. Although this method provides corrections to sensor input caused by flow sensor drift and offset, it still suffers from serious offsets in the leak detection signal during changes in process parameters, namely steaming rate changes.
Consequently, there remains a need in the art for more flexible leak detection methods which can be employed in boiler systems that are not at steady state, that is, where one or more process parameters is subject to change.